1. Field of the Invention
This invention relates to a system for and a method of controlling a resistance welder for welding workpieces clamped between two electrodes with Joule heat generated by passing a welding current between the electrodes.
2. Description of the Prior Art
For obtaining satisfactory welding with a resistance welder, an adequate welding current has to be passed between two electrodes in dependence on workpieces. Japanese Patent Publication No. 62-18273 discloses a prior art system for automatically causing adequate welding current. In this system, a desired resistance curve representing the resistance between two electrodes that varies with time is set, and the welding current that flows between the electrodes is controlled such that the actual resistance between the electrodes conforms to the desired resistance curve.
The desired resistance curve varies depending on such welding conditions as the quality, thickness, quantity, etc. of the workpieces. That is, in the prior art system described above, the optimum resistance curve varies depending on the welding conditions such as the material of the workpieces. Therefore, it is necessary to set a desired resistance curve (reference resistance values) for each welding condition, which is considerably cumbersome.
Further, in order to perform adequate welding, the welding current has to be controlled according to the rate of expansion or resistance of workpieces as described later. Generally, the rate of expansion or resistance of workpieces is judged on the basis of the distance or the resistance between the electrodes. In the following case, however, the rate of expansion or resistance of the workpieces and the distance (or resistance) do not correspond to each other, so that adequate control cannot be obtained.
In case where the electrodes are not flat but have a projection or the like resulting from the breakage of their tip due to their use for a large number of times, in an initial stage of welding, such projection of the electrode sinks into the workpiece, so that the electrodes correspondingly approach each other. Consequently, even when the workpieces are inflated by heat generated by passing current, a certain portion of the expansion is canceled by the sunk portion of the electrode. Therefore, when the rate of expansion of the workpieces is judged from the distance between the electrodes, it is judged to be less than the actual rate of expansion. For this reason, if prior art control is directly adopted, it results in causing a greater welding current than is necessary, thus resulting in spattering.
A similar situation results where foreign particles are present between electrode and workpiece. In this case, as the foreign particles are burnt or dissolved by heat generated by current passing, the electrode is moved further toward the workpiece. Therefore, when the judgment is made from the distance between the electrodes, the work is judged to be shrunk correspondingly, and the rate of expansion of the workpieces is judged to be less than the actual rate of expansion.
In the above two cases, "the rate of expansion of the workpieces" may be replaced with "the resistance of the workpieces".